This application claims the priority benefit of Taiwan application serial no. 89102591, filed Feb. 16, 2000.
1. Field of the Invention
The present invention relates to a device and method applied in an optical storage device for focusing on an optical disk. More particularly, the present invention relates to a device and method capable of finding a focus on an optical disk by utilizing a high-pass filter and a focus zero cross level.
2. Description of Related Art
FIG. 1A is a schematic side view of a pickup head and a disk in an optical storage device. An optical disk 10 is placed inside an optical disk tray. A motor (not shown) drives the optical disk 10 driven into rotation (not shown). Meanwhile, a pickup head 20 moves towards the disk surface in search for a focus. After the pickup head 20 has properly focused on the disk surface, data is read from the optical disk 10. The optical storage device has a radio frequency amplifier (RF Amp.) for sending out a focus error (FE) signal. The focus error signal serves to decide whether the pickup head 20 is in focus or not.
FIG. 1B is a graph showing the variation of focus error signal sensed by the radio frequency amplifier when the pickup head approaches the optical disk shown in FIG. 1A. As the pickup head 20 approaches the optical disk 10, the radio frequency amplifier generates an S-curve in the neighborhood region of the focus. As shown in FIG. 1B, the focus error signal rises from zero to a peak when the head 20 moves close to the optical disc 10. Then the focus error signal drops rapidly passing through a zero mark and reaches a bottom value (this region is a linear section). Finally, the focus error signal bounces back from the bottom to almost zero if the pickup head 20 still approaches the optical disk 10. Focus for the pickup head 20 is located at the position where the focus error signal in the linear section is zero.
FIG. 1C is a graph showing the variation of focus error signal sensed by the radio frequency amplifier when the pickup head moves away from the optical disk. Similarly, when the pickup head 20 moves away from the optical disk 10 from a close distance, the radio frequency amplifier generates an S-curve in the neighborhood of the focus. As shown in FIG. 1C, the focus error signal drops from zero to a bottom when the optical head 20 moves away from the optical disk 10. Then, the focus error signal rises rapidly passing through a zero mark and reaches a peak (this region is a linear section). Finally, the focus error signal drops again from the peak value to almost zero when the pickup head 20 still moves far away from the optical disk 10. Focus for the pickup head 20 is located at the position where the focus error signal in the linear section is zero.
Ideally, the aforementioned method is capable of finding a proper focus for the pickup head. However, the optical disk is likely to wobble in rotation. Consequently, the focus error signal will be incomplete and position of the actual focus will be difficult to determine.
FIGS. 2A and 2B are graphs showing incompleteness in tracing out the frequency error signal when the pickup head approaches a wobbly optical disk. In FIG. 2A, focus error signal starts to drop before reaching a peak. Instead of a rapid drop, the drop in focus error signal is slow. Moreover, there is no further drop as soon as the focus error signal reaches zero. In FIG. 2B, focus error signal also drop before reaching a peak. However, there is another rise and fall after the first one. Under both conditions shown in FIGS. 2A and 2B, the optical system will regard the point having zero focus error value as the focus. Hence, the pickup head may be out of focus and data read from the optical disk may be in error.
In addition, the focus error signal generated by the radio frequency amplifier may also produce a shift in DC level. Due to a shift in the DC level, position of zero point in the trace for focus error signal may be incorrectly determined and may result in improper focusing.
To combat the above problems, a high-pass filter (HPF) is normally added to a conventional optical storage device. Since the main function of a high-pass filter is to filter out the DC level component within focus error signal, the high-pass filter has a cutoff frequency greater than zero. The high-pass filter is also capable of producing a focus error high-pass filter (FE-HPF) signal and providing a focus on level (FONL) for comparing with the focus error high-pass filter signal. After the focus error high-pass filter signal is higher and then lower than the FONL signal, position where the FE-HPF signal reaches zero again is the focus of the pickup head.
Although the introduction of a high-pass filter to a conventional system solves some error focusing, errors in focusing still happens. FIGS. 3A, 3B and 3C show error waveforms generated in searching a focus on the optical disk. In FIG. 3A, a normal focus error high-pass filter signal is shown. The zero point in the linear section of the graph is a correct focus for the pickup head. In FIG. 3B, the graph of an abnormal focus error high-pass filter signal is shown. Since this abnormal FE-BPF signal is not higher than the FONL signal, erroneous focusing will not occur. In FIG. 3C, another abnormal focus error high-pass filter signal is shown. However, because the focus error high-pass filter signal is higher and then lower than the FONL signal, and drops back to zero level, the zero-crossing position will be interpreted as a proper focus for the pickup head leading to possible data-reading errors.
Accordingly, one object of the present invention is to provide an installation and method for focusing on an optical disk. The method includes passing a focus error signal through a high-pass filter to generate a focus error high-pass signal in addition to providing a focus on level. The criterion for finding the focus on the optical disk is the observation that the focus error high-pass signal has reached the focus on level.
A second object of this invention is to provide a device and method capable of finding the correct focus between an optical disk and a pickup head so that improper focusing is prevented.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an installation for finding a correct focusing distance to an optical disk. A device for finding the focus between an optical pickup head and an optical disk by tapping signals from a radio frequency amplifier comprises a high-pass filter for receiving a focus error signal from the radio frequency amplifier and generating a focus error high-pass signal; and a decision circuit that couples to the high-pass filter for receiving the focus error high-pass signal and using a focus zero cross level as a reference for determining a correct focusing point. The focus between the optical pickup head and the optical disk is found when the focus error high-pass signal reaches the positive focus zero cross level or the focus error high-pass signal reaches the negative focus zero cross level.
The invention also provides a device for finding the focus between an optical pickup head and an optical disk by tapping signals from a radio frequency amplifier. A high-pass filter for receiving a focus error signal from the radio frequency amplifier and generating a focus error high-pass signal; and a decision circuit. The decision circuit further comprises a subtraction unit for subtracting the focus zero cross level from the focus error high-pass signal and outputs the result of the subtraction; an adder unit for adding the focus zero cross level to the focus error high-pass signal and outputs the result of the addition, and a control circuit for monitoring the results from the subtraction unit and the adder unit so that the correct focus point is found when output from either the subtraction unit or the adder unit is zero.
This invention also provides a method for finding the focus between an optical pickup head and an optical disk. First, a focus error signal is provided from the pickup head/optical disk system. A high-pass filtering on the focus error signal is performed to produce a focus error high-pass signal. A focus zero cross level signal is further provided. Finally, the focus point is captured when the value obtained by subtracting the focus zero level from the focus error high-pass signal or adding the focus zero level to the focus error high-pass signal is zero.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.